1. Field of the Invention
The present invention relates to a photonic crystal that is useful in the fields of photonics and electromagnetic waves, to a method of producing the photonic crystal, and to a functional element utilizing the photonic crystal, such as a laser element, an optical switch, and a tunable filter.
2. Description of the Related Art
Refractive index periodic structures having a distribution in which the refractive index is periodic exhibit a diffractive/interfering action with respect to electromagnetic waves, and prohibit the propagation of electromagnetic waves of specific frequency bands. This phenomenon corresponds to band structures with respect to electrons in a semiconductor crystal. Generally, such refractive index periodic structures are called photonic crystals, and frequency bands that prohibit propagation are called photonic band gaps. The information technology industry that flowered at the end of the twentieth century was supported by electronics based on semiconductor materials that control electrons, but it is nearing an inherent technological limit. It is thought that, in order for further development in the twenty-first century, a move to photonics that can break through the limit of electronics is essential. Because photonic crystals can control electromagnetic waves, they are ranked as key materials in photonics similar to the way semiconductors are in electronics, and are promising as an important element for realizing next-generation optical devices such as ultra-efficient lasers and ultra-miniature optical integrated circuits.
In order for photonic crystals to function effectively, the photonic crystals must have a refractive index periodic structure of a spatial scale similar to that of the wavelength of the electromagnetic wave that is to be controlled, and it is necessary for the ratio of refractive index between a high refractive index phase and a low refractive index phase to be equal to or greater than a predetermined value. The lowest refractive index ratio sought differs in accordance with the configuration of the periodic structure, but generally the larger it is, the more preferable it is. In the field of photonics, because the target wavelength region is generally from the visible light region to the near infrared region, photonic crystals having a period from a submicron order to micron order must be created. As a method for realizing this, an example has been disclosed by Lin et al in which semiconductor microfabrication technology is used to create a woodpile-like photonic crystal in which blocks made of Si are stacked at periods of several microns (Nature, Vol. 394, pp. 251–253 (1998)). Also, wafer fusion has been disclosed by Noda et al as a method for creating a woodpile-like photonic crystal, in which blocks made of GaAs and InP are stacked at periods of several microns is created (App. Phys. Lett., Vol. 75, pp. 905–907 (1999)). Additionally, Kawakami et al have succeeded in the creation of a photonic crystal, by unique bias sputter deposition/etching, having a special 3-dimensional periodic structure of a submicron order comprising Si and SiO2 (Electron. Lett., Vol. 33, pp. 1260–1261 (1997)), and they have called this method self-cloning. Furthermore, Vos et al have created an inverse opal photonic crystal of a submicron order by depositing, by sol-gel, titania into the pores of an opal structure resulting from the self-assembly of polystyrene monodisperse particles, and removing the polystyrene particles of the mold by burning them at the same time as baking the titania (Science, Vol. 281, pp. 802–804 (1998)). Misawa et al have created, by 2-photon polymerization, a woodpile-like photonic crystal of a submicron order comprising a photocurable resin (Appl. Phys. Lett., Vol. 74, pp. 786–788 (1999)).
However, because Lin et al's method comprises many steps that combine complicated semiconductor microfabrication technology, there are problems in that a large apparatus is necessary, productivity is low, and costs are high. There are also few types of applicable materials, and the method cannot be said to be versatile. Noda et al's method is an extremely excellent method in that there are many types of applicable materials and there is a great amount of freedom with respect to structure. However, the extremely harsh condition of heating at about 700° C. in a hydrogen atmosphere is used in order to conduct wafer fusion, and there are problems with safety in the fabrication and the like. Kawakami et al's method is extremely excellent in that there are many types of applicable materials. However, there are serious problems in that some limited types of structures can be created and the method lacks versatility. Because opal and inverse opal photonic crystals are extremely simple to create, they are widely used in research activities at the laboratory level, but the amount of freedom with respect to structure is small, and breakthroughs in terms of production methods are essential when using them in a device. From theoretical calculations, the refractive index conditions necessary to form complete photonic band gaps in opal and inverse opal photonic crystals are predicted to be remarkably more severe than those necessary for woodpile-like photonic crystals, and are disadvantageous in terms of material selectability. With respect to inverse opal photonic crystals, it is necessary to fill a high refractive index material into the pores of the opal mold. However, there are problems in that it is difficult to evenly fill fine 3-dimensional pores and the mold becomes deformed in accompaniment with the filling. As a method for creating a photonic crystal using a photocurable resin, a method using ordinary optical molding has also been proposed in addition to the above-described 2-photon polymerization. Because the refractive index of the resin in the method using a photocurable resin is about 1.7 at best, which is low, there has been the problem that a large refractive index ratio cannot be obtained. In the method using 2-photon polymerization, an extremely expensive femtosecond laser must be used, and there is the problem that the method is ill-suited for commercial production. Optical molding is a method that is practically utilized for rapid prototyping in production processes for household electrical appliances, but resolution in current machines is low, and it impossible to use in the creation of photonic crystals of an optical wavelength region.
In recent years, functional materials have been incorporated into photonic crystals. Using reaction characteristics of functional materials with respect to an outside stimulus, there have been attempts to add new functions to photonic crystals to develop the photonic crystals into functional elements. For example, a photonic crystal has been 7 created by Busch et al, in which a liquid crystal material whose refractive index is changed in response to an electric field or heat is filled into the pores of an inverse opal photonic crystal (Phys. Rev. Lett., Vol. 83, pp. 967–970 (1999)). This photonic crystal can control the opening and closing of the photonic band gap by the application of an electric field, and can be applied as a functional element such as an optical switch or an imaging element. A photonic crystal has also been created by Meier et al, in which a light-emitting material that emits fluorescent or phosphorescent light due to light absorption is filled into the surface and pores of a honeycomb photonic crystal (J. Appl. Phys., Vol. 86, pp. 3502–3507 (1999)). This photonic crystal can be used as a photoexcitation laser element by the photonic crystal structure functioning as a resonator.
However, in each of the above-described cases, a method is adopted in which the functional material is filled after the photonic crystal structure has been created, but because it is difficult to homogenously fill the fine 3-dimensional pores of the photonic crystal structure with the functional material, the development of a production method that is simple and whose productivity is high has been sought after with the hope of practical application.
A versatile method of producing a photonic crystal having a desired refractive index and periodic structure, and a simple method of producing a photonic crystal that incorporates a functional material, have not been established yet. Therefore, photonic devices utilizing photonic crystals have not yet reached full-scale practical utilization.